1. Field of the Invention
This invention relates to assemblies for transmitting torque between a shaft and a hub or wheel mounted thereon. More particularly, the invention relates to such assemblies in which the hub or wheel is threaded onto the shaft and locking capability is attained through the provision of a unique thread shape. In its broader aspects, the invention relates to any assembly of a female member threaded onto a male member in which the members must be locked against rotation relative to each other.
2. Description of the Prior Art
Assemblies comprising a threaded cylindrical male member and a female member having a cylindrical bore threaded to match the threads in the male member are used for many applications in which it is desirable, and frequently critical, that under certain conditions the female member be locked against rotation with respect to the male member; in these applications, it is also frequently desirable that the "lock" be reversible to allow the members to be separated when necessary. A simple common example of such an assembly is that of a bolt and nut in which a lock washer between the nut and the surface to be held in place prevents loosening of the nut when the assembly is tightened.
In cases where torque is to be transmitted between a shaft and a hub or wheel mounted thereon, such as where a hub is mounted on the end of a driving shaft for connection through a flexible coupling to a similar hub mounted on the end of a driven shaft, or where a turbine wheel is mounted on a shaft, any of several attachment means have heretofore been used; however, each presents problems.
In the prior threaded shaft connection, which is seldom used, a threaded shaft end is screwed into a blind hole in a hub or wheel until it is tight against the bottom of the hole. This arrangement is sometimes useful, but can primarily transmit torque in only one direction; for example, if a right hand thread is used, the hub or wheel can be released if torque is applied in the left hand direction, a distinct disadvantage because reverse torques sometimes occur.
A straight keyed shaft connection comprises keyways cut into the shaft and into the side of the central hole of the hub or wheel; the keyways are radially aligned and a key is inserted to lock the hub or wheel to the shaft. In high speed and high torque applications where extremely tight fits are important, the required tightness is usually achieved by making the bore in the hub or wheel slightly smaller than the diameter of the shaft, expanding the hub or wheel by heating it, slipping it onto the shaft, and allowing it to cool, thereby resulting in a very tight interference fit on the shaft. This type of connection has several disadvantages. For one, the keyway cut into the shaft produces high stress concentrations which are aggravated with the use of heat for shrink fitting the hub or wheel on the shaft. The need for precise tolerances in the keyways makes manufacturing difficult and frequently time consuming. Where heat is used to produce a shrink fit, the torches or special heaters required can present a hazard depending on the environment in which the assembly is carried out.
Tapered key shaft hubs are used because by making a taper in the bore, a hub can be tightened on a shaft by pulling up on a nut threaded onto an extension of the shaft which protrudes past the hub. However, it is extremely difficult to get a tapered bore accurate and to match bores between shaft and hub, making this type of connection expensive and difficult. Further, a tapered shaft hub also has the disadvantage that stress concentrations can occur in the keyway, greatly reducing the effective strength of the connection.
Both types of keyed shaft connections generally require torch heating for removal.
A tapered shaft hydraulic connection is assembled by applying high hydraulic pressure in circumferential channels formed on the inside of a hub or wheel opening, thereby "stretching" the hub or wheel; when the hub or wheel is in place, the pressure is released, which effectively shrinks the hub or wheel on the shaft. While this system sometimes eliminates the need for a keyway, it transmits torque purely by friction, which can be unreliable. It also requires extreme accuracy in manufacturing the members of the connection and special hydraulic pumps, equipment and seals for assembly. This type of connection is very expensive and not often used.
Splined connections, in which a shaft and the opening in a hub or wheel are splined to mate when the hub or wheel is fitted on the shaft end, are sometimes used for connections of the type being discussed, but they are expensive to fabricate, since forming of the splines requires special high precision machinery.
In view of the foregoing, it can be seen that, especially in the field of shaft-mounted hubs and turbine wheels, a need exists for an improved connection assembly. Such an assembly should preferably include the following features:
1. It requires minimum skills and no special equipment to construct; PA1 2. The hub can be mounted on the shaft without the need for high temperatures in heating equipment; PA1 3. The assembly transmits maximum torque without creating high stress concentrations in the shaft; PA1 4. The hub mounts symmetrically to the shaft centerline, particularly important in high speed machinery sensitive to unbalanced weight on the shaft; PA1 5. The hub can be easily adjusted in axial position on the shaft; PA1 6. The hub can be easily removed with simple tools and without heating; PA1 7. The assembly parts can be easily manufactured--i.e., without excessively tight tolerances and with a minimum of required machinery setups; and PA1 8. The assembly can transmit torque in either direction.
The foregoing and other advantages can be attained with the threaded assembly of my invention.